1. Field of the Invention
The present invention generally relates to a mobile communication system, and more particularly to a mobile communication terminal which detects a transmission bit rate that is selected at a base station and a transmission-bit-rate detection method.
In the field of mobile communication, access methods which assign maximum users to limited frequency resources are desired. These access methods include, for example, Frequency-Division Multiplex Access (FDMA), Time-Division Multiplex Access (TDMA) and Code-Division Multiplex Access (CDMA). In the FDMA and the TDMA, one radio station occupies one radio channel and slot. On the other hand, in the CDMA, a wide-band radio channel is shared among many users by adding proper code to a signal.
Presently, the mobile communication systems which employ the FDMA and the TDMA are in practical use. On the other hand, the mobile communication system which employs the CDMA is desired because it is robust against both interference and disturbance and can keep privacy. This is because the proper code is added to the signal in the CDMA.
2. Description of the Related Art
A conventional transmission-bit-rate detection method which detects a transmission bit rate that is selected at a base station for a conventional mobile communication system is explained below.
Generally, when the base station communicates with a mobile communication terminal, speech data and control data are transmitted through a speech channel in the mobile communication system which employs the CDMA. The speech channel can carry the speech data and the control data at any bit rate among 9.6 kbps, 4.8 kbps, 2.4 kbps and 1.2 kbps. The data is transmitted at a variable transmission bit rate using these four bit rates.
The base station adds error detection code, for example, Cyclic Redundancy Check (CRC), and tail bits to information bits of the speech channel. The CRC is only added when the transmission bit rate is either 9.6 kbps or 4.8 kbps. Next, the base station adds convolutional code for error correction to the information bits of the speech channel to which the CRC and the tail bits are added and generates transmission symbols. Then, the base station outputs the transmission symbols at the designated bit rate among 9.6 kbps, 4.8 kbps, 2.4 kbps and 1.2 kbps. When the designated bit rate is 9.6 kbps, then transmission symbols are output one time. When the designated bit rate is 4.8 kbps, then transmission symbols are repeatedly output twice. When the designated bit rate is 2.4 kbps, then transmission symbols are repeatedly output three times. When the designated bit rate is 1.2 kbps, then transmission symbols are repeatedly output four times. This is because the same number of bits is needed to detect the transmission bit rate at the mobile communication terminal.
Next, these transmission symbols are interleaved and scrambled with long Code which is a user identification code and enables synchronization at the mobile communication terminal. Then, power control bits are added. The power control bits control strength of a radio wave for each mobile communication terminal. A weak radio wave is transmitted to the mobile communication terminal near the base station and a strong radio wave is transmitted to the mobile communication terminal far from the base station to equalize the strengths of the radio waves received by the mobile communication terminals. After all of the processes described above are ended, the base station spreads spectrum of the transmission symbols over a wide band. Next, the base station modulates the transmission symbols and transmits the transmission symbols.
The mobile communication terminal of the mobile communication system which employs a spread spectrum method operates as follows to detect the transmission bit rate.
FIG. 1 shows main parts used to detect the transmission bit rate for the conventional mobile communication terminal.
At the mobile communication terminal, first, received data is demodulated and its spectrum is inverse-spread. Next, long Code is generated based on information from a sync channel and inverse spectrum spread data is descrambled using the information. Then, descrambled data is deinterleaved. As a result, received symbols are reproduced. The received symbols are supplied to four Viterbi decoders, such as a 9.6-kbps Viterbi decoder 101, a 4.8-kbps Viterbi decoder 102, a 2.4-kbps Viterbi decoder 103 and a 1.2-kbps Viterbi decoder 104, which are connected in parallel as shown in FIG. 1. Each decoder executes Viterbi decoding at each bit rate and outputs decoded data.
Convolutional re-encoders, such as a 9.6-kbps convolutional re-encoder 105, a 4.8-kbps convolutional re-encoder 106, a 2.4-kbps convolutional re-encoder 107 and a 1.2-kbps convolutional re-encoder 108, re-encode the decoded data. A selector 109 compares the data which is re-encoded by the convolutional re-encoders with the received data. Then, the selector 109 detects the transmission bit rate based on comparison results which have minimum errors. The bit rate of the convolutional re-encoder which outputs re-encoded data with minimum errors is the transmission bit rate and the decoded data of the Viterbi decoder which has the same bit rate as that of the convolutional re-encoder with minimum errors is output to a codec. The conventional mobile communication terminal detects the transmission bit rate described above.
However, in the conventional mobile communication terminal, the four Viterbi decoders, consisting of the 9.6-kbps Viterbi decoder 101, the 4.8-kbps Viterbi decoder 102, the 2.4-kbps Viterbi decoder 103 and the 1.2-kbps Viterbi decoder 104, shown in FIG. 1 decode the received data simultaneously at all bit rates. Moreover, the four convolutional re-encoders, consisting of the 9.6-kbps convolutional re-encoder 105, the 4.8-kbps convolutional re-encoder 106, the 2.4-kbps convolutional re-encoder 107 and the 1.2-kbps convolutional re-encoder 108, re-encode the decoded data simultaneously at all bit rates.
In the conventional mobile communication terminal, the Viterbi decoding and the convolutional re-encoding at all bit rates are executed simultaneously. Therefore, this results in both increase of a circuit scale and increase of consumption power.
It is a general object of the present invention to provide a mobile communication terminal in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a mobile communication terminal which enables a small-sized mobile communication terminal based on reduction of the circuit scale and reduction of the consumption power of the mobile communication terminal.
The above objects of the present invention are achieved by a mobile communication terminal which receives convolutionally encoded data that is convolutionally encoded information of a speech channel transmitted from a base station and detects a transmission bit rate selected at the base station by decoding the data. The mobile communication terminal comprises a rate estimation unit which estimates the transmission bit rate selected at the base station and outputs an estimated transmission bit rate, a decoding unit which decodes the convolutionally encoded data transmitted from the base station and outputs decoded data and predetermined types of results of decoding, a convolutional re-encoding unit which convolutionally re-encodes the decoded data and outputs re-encoded data, and a rate detection unit which detects whether the estimated transmission bit rate is correct or not based on the decoded data and the predetermined types of results of decoding.
As the transmission bit rate is estimated by the mobile communication terminal, a plurality of decoders for all bit rates is not necessary. A plurality of re-encoders is also not necessary for the same reason. Therefore, the simultaneous decoding of the received data by four decoders at all bit rates and the simultaneous re-encoding of the decoded data by four re-encoders at all bit rates are not necessary. This results in the small-sized mobile communication terminal based on the reduction of the circuit scale and enables the reduction of the consumption power of the mobile communication terminal.
The above objects of the present invention are achieved by a transmission-bit-rate detection method for a mobile communication terminal which receives convolutionally encoded data that is convolutionally encoded information of a speech channel transmitted from a base station. The transmission-bit-rate detection method comprises a rate estimation step which estimates the transmission bit rate selected at the base station and outputs an estimated transmission bit rate, a decoding step which decodes the convolutionally encoded data transmitted from the base station and outputs decoded data and predetermined types of results of decoding, a convolutional re-encoding step which convolutionally re-encodes the decoded data and outputs re-encoded data, and a rate detection step which determines whether the estimated transmission bit rate is correct or not based on the decoded data and the predetermined types of results of decoding.
As the transmission bit rate is estimated by the rate estimation step, a plurality of decoders for all bit rates is not necessary. A plurality of re-encoders is also not necessary for the same reason. Therefore, the simultaneous decoding of the received data by four decoders at all bit rates and the simultaneous re-encoding of the decoded data by four re-encoders at all bit rates are not necessary because the transmission-bit-rate detection method is executed. This results in the small-sized mobile communication terminal based on the reduction of the circuit scale and enables the reduction of the consumption power of the mobile communication terminal.